The ignitability of spark plugs has been increased by, for example, increasing the ignition energy applied to the spark plug (see Japanese Unexamined Patent Application Publication No. 2013-108465).
However, to increase the ignition energy, an ignition coil or the like capable of generating a large amount of energy is required, and therefore the cost of the entire ignition system is increased.
Accordingly, a technology for increasing the ignitability of a spark plug with a simple structure is in demand.
The present invention has been made to solve the above-described problem, and can be embodied in the following forms.